A mathematical technique for using satellite images to gauge the growth rate and mass of the ocean's phytoplankton, or plant plankton, could help scientists more fully understand the food cycles of the oceans, as well as global climate change, researchers say.

The new approach--devised by researchers with NASA, the University of California, Santa Barbara, and other institutions--is described in a paper published recently in the Global Biogeochemical Cycles journal.

The method involves studying the "greenness" and brightness of phytoplankton from satellite images. Computers take the data from the images, and then try to mathematically determine the levels of phytoplankton and the rate of reproduction taking place.

Today's satellite technology cannot fully take advantage of the new method, but researchers say that some benefits can be gleaned. Also, researchers on the project are already working on an instrument, called the Ocean Radiometer for Carbon Assessment, that will let satellite systems better determine color and brightness.

Although often dismissed as unicellular scum, phytoplankton constitutes the foundation of ocean life. About 50 billion to 65 billion tons get produced annually, as much as the net annual production of terrestrial plant life. Fish and animals feed on the plants, which also play a crucial role in absorbing carbon dioxide and producing oxygen.

Scientists figured out how to determine the world's biomass of phytoplankton in the 1950s, but the new techniques will allow researchers to study the rate of production over broad areas in a somewhat dynamic fashion.

The changes in phytoplankton in a region can be dramatic: The biomass of phytoplankton in a given area can double in a day. Conversely, the entire mass in a region can be eaten or sink to the bottom of the ocean in a week.

The health of local fisheries largely revolves around what happens to these biomasses.

"The growth rate of phytoplankton can change dramatically based on such factors as water temperature, nutrients and light," said Michael Behrenfeld, an author of the paper and a professor in the department of botany at Oregon State University. "It's the growth rate of phytoplankton we have to know (in order) to really take the pulse of the oceans."